1. Field of the Invention
The present invention relates to a cover composition for a golf ball.
2. Description of the Related Art
There has been known, as a cover for a golf ball, balata cover, ionomer cover, and urethane cover. Among these, the urethane cover enjoys prestige in light of the merits that the golf ball having the urethane cover provides good shot feeling and controllability inherent to the balata cover, as well as durability and superior flight performance inherent to the ionomer cover.
The golf ball having the urethane cover, however, is likely to cause discoloration such as yellow discoloration with time, because it is used outdoors. This is because the urethane composing the cover is exposed to UV rays of the sunlight, and resultantly degraded.
There is known a method of using an aliphatic/alicyclic polyisocyanate having good weatherability as a raw material for the urethane cover to keep the urethane cover from causing yellow discoloration.
Japanese Unexamined Patent Publication No. 2002-159596 discloses that a urethane composition for a cover contains a color stabilizer such as a UV absorber and a light stabilizer.
Use of the aliphatic/alicyclic polyisocyanate as a material for the urethane cover enables to improve weatherability, as compared with a case of using an aromatic polyisocyanate. However, the effect by use of this component is not sufficient because the polyurethane cover causes yellow discoloration not only due to degradation of the urethane by exposure to UV rays but also due to degradation of the urethane arising from photocatalytic action of titanium oxide. Such titanium oxide is generally contained in the cover composition as a white pigment for providing whiteness to the golf ball.
Specifically, by exposure to UV rays, an electron-hole pair consisting of a free electron (e−) on a conduction band and a positive hole (p+) on a valence electron band is generated in the titanium oxide due to excitation of the electron on the valence electron band, as represented by the following formula (1).TiO2+hr→p++e−  (1)
The positive hole (p+) is entrapped by OH− in adsorbed water on the surface of the titanium oxide, thereby generating an OH free radical, as represented by the formula (3). The generated free radical decomposes a resin component of the urethane cover by its powerful oxidizing action.H2O→H++OH−  (2)OH−+p+→{dot over (O)}H  (3)
The free electron (e−) reduces Ti4+ in a crystal of the titanium oxide to Ti3+ as represented by the formula (4). Subsequently, O2 in the air is adsorbed to the Ti3+, and turns to O−2 as represented by the formula (5). Then, the O−2 is reacted with the H+ in the formula (2), and turns to an H{dot over (O)}2 free radical while undergoing the reaction as represented by the formula (6). Similarly to the OH free radical, the H{dot over (O)}2 free radical decomposes the resin component of the urethane cover around the titanium oxide by its powerful oxidizing action.[Ti4+]+e−→[Ti3+]  (4)[Ti3+ ]+O2→[Ti4+]+O−2  (5)O−2+H+→H{dot over (O)}2  (6)
As mentioned above, there is a drawback that weatherability of the cover cannot be improved sufficiently, as far as the cover composition contains titanium oxide, despite inclusion of a color stabilizer such as a UV absorber and a light stabilizer, as disclosed in the above publication.